1. Field of Invention
The invention relates generally to wastewater treatment and more particularly to the establishment of flocculated bacteria to wastewater treatment systems.
2. Prior Art
Bacteria are utilized in wastewater treatment to break down and remove pollutants from the water before it is discharged. One of the ways the pollutant levels are measured is by testing five day biological oxygen demand (BOD.sub.5) of the effluent. Another test criteria is total suspended solids (TSS) present in the effluent. This criteria is typically referred to as the mixed liquor suspended solids (MLSS) when it is measured in the biological treatment lagoon or aeration tank. The organic fraction of the suspended solids are referred to as mixed liquor volatile suspended solids (MLVSS), and is generally about 60-85% of the MLSS. To satisfy local and federal regulations which both vary from plant to plant, effluent must be within certain BOD.sub.5 and TSS limits among other criteria. Failure to satisfy these regulations can have various adverse consequences, including fines for the plant and criminal penalties for the plant management. Exceeding discharge levels can also have adverse environmental effects on the waterways into which the effluent is discharged.
Bacteria in wastewater treatment plants can be loosely grouped into three principal categories for purposes of the present invention: dispersed bacteria; flocculated bacteria; and filamentous bacteria. As the name implies, dispersed bacteria are isolated bacteria that are free in the wastewater. The other two categories describe bacteria that exist in large groups or `flocs` containing many bacteria. The higher surface area of the flocculated bacteria allows them to compete more effectively for the organic matter that contribute to the BOD.sub.5 of the wastewater. As the floes capture more and more organic matter, their density increases, and in typical floes, they eventually fall out of solution, forming a sludge and leaving a supernatant that is low in both TSS and BOD.sub.5.
Filamentous bacteria form flocs that are quite different from those of the flocculated bacteria. The filamentous floc is comprised of numerous long filaments that extend between the bacteria. These filaments can be several times as long as the bacteria and there can be several filaments produced by each bacteria. The end result is a filamentous floc that is high in surface area and low in density. The low density prevents the filamentous bacteria from settling out of the wastewater. When the effluent is discharged, the filamentous flocs add to the suspended solids count and can cause the effluent to exceed its TSS permit level.
One way that the `settleability` of the sludge is measured is by the sludge volume index or SVI, measured in milliliters per gram. An SVI value of below 100 ml/g is a good indication that the sludge will settle and compact well. Conversely, an SVI value above 200 ml/g indicates that the sludge will settle slowly and that a coagulant will probably be needed. High SVI levels are one indication that there may be a filamentous infestation in the wastewater. A good explanation of SVI is provided in The Nalco Water Handbook, 23.1-23.22 (Frank N. Kemmer ed., McGraw-Hill 1988)(2nd ed.) which is hereby incorporated by reference to the extent it is not contrary to the teachings herein.
The degree of filamentous bacteria infestation is rated on the Eickelbloom, Jenkins and Richards scale which ranges from 0 to 6, with 6 being the highest level of infestation and 0 being completely free of filamentous bacteria. Filamentous infestations that rate in excess of 4 will usually interfere with settling.
Certain conditions favor the establishment of filamentous bacteria in the wastewater stream. This typically arises when dissolved oxygen content is low (less than about 0.1 to 0.5 mg/l), when the levels of nutrients such as NH.sub.3 and PO.sub.4 are low (less than about 1 mg/l each), when sulfide content is high, and when food to microorganism ratios (F/M ratios) are either very high (above about 0.5 lb. BOD.sub.5 per lb. MLSS) or very low (below about 0.2 lb. BOD.sub.5 per lb. MLSS). These causative conditions should be corrected if possible.
Once filamentous bacteria become established, it can be very difficult for the more desirable flocculated bacteria to displace them. The high surface area of the filamentous bacteria makes them very effective in terms of their ability to compete with the other bacteria for organic matter, oxygen, and nutrients suspended in the wastewater.
One solution is to add chlorine to the system, often in the form of NaClO. Most common sources of chlorine (e.g., Cl.sub.2, NaClO, and ClO.sub.2) form hypochlorous acid (HClO) when they dissociate in water. Hypochlorous acid is a recognized disinfectant which works by disrupting transfers across the bacteria cell walls. The chlorine indiscriminately kills the bacteria that it contacts. This can create a problem because it is undesirable to eliminate all of the bacteria. Some bacteria must remain to continue treating the wastewater. Elimination of all of the bacteria will likely result in the effluent exceeding discharge limits for BOD.sub.5 and other criteria.
The goal in chlorination is usually to eliminate only the filamentous bacteria so that the flocculated bacteria can reestablish predominance. This is done by providing the chlorine in low doses. The idea is that the high surface area of the filamentous bacteria, which helps it out compete the other bacterial forms for organic matter, oxygen, and other nutrients will also cause it to absorb more of the chlorine. Thus, a larger percentage of the filamentous bacteria will be killed by the chlorine relative to the other bacteria. The remaining flocculated bacteria, having a larger surviving population will, in theory at least, be able to predominate as the bacterial population reestablishes itself.
Several problems commonly arise in chlorination treatment. One of the most common is over-chlorination. The difference between the quantities needed to eliminate the filamentous population and the quantities that will wipe out substantially all of the bacteria can be relatively small. Providing too much chlorine can reduce the bacterial population so severely that there are not enough microorganisms left to treat the wastewater as discussed above.
Another problem is that killing the filamentous bacteria may not keep them from returning. The conditions which fostered the growth of the filamentous bacteria in the first place cannot always be easily corrected. If favorable conditions for the filamentous bacteria continue to exist in the treatment stream, new filamentous bacteria will continue to appear, and may predominate as the post-chlorination bacterial population is reestablished.
Another problem that can arise is chlorine resistant bacteria. The practice of attempting to kill the filamentous bacteria with the smallest amount of chlorine possible will invariably lead to the survival of some of the bacteria. These bacteria will pass on their chlorine resistant traits to their offspring, creating a population of filamentous bacteria that is chlorine resistant. If the filamentous problem persists in the treatment plant, it can become increasingly difficult to effectively control them with limited quantities of chlorine. As the required dosages increase, so does the likelihood that the flocculated bacteria will be killed along with filamentous bacteria.
After the filamentous bacteria have been inhibited, it is still necessary to get the fragments of the filamentous floes and the other suspended solids to settle out to avoid violating TSS discharge limits. Many suspended particles are colloids which can make settling difficult.
A colloid is a particle of one substance that is surrounded by particles of a second substance such that the particles of the first substance are prevented from combining. Suspended particles that have a diameter of less than about 10 .mu.m are generally considered colloidal. Most bacteria have a diameter of about 1 .mu.m and thus are considered colloidal. Gravity is always operating to pull the colloidal particles down and out of suspension. Opposing the force of gravity are the van der Waals forces which keep the particles in suspension and separated from one another. The principle van der Waals force in an aqueous solution is the interaction between the dipole of water molecules and the negative charge that is present on most colloidal and other suspended particles. To get the particles to fall out of suspension, the first step is to eliminate the charge on the particles.
One type of coagulant used to remove colloidal particles from solution are metal salts such as alum, lime, ferric chloride, and ferrous sulfate. The salts release metal ions upon dissolution, which then form hydroxides at varying rates depending upon the pH of the system. For example, alum, Al.sub.2 (SO.sub.4).sub.3, forms a water insoluble aluminum hydroxide, Al(OH).sub.3, when added to water. The wastewater stream is usually agitated to bring the hydroxide particles into contact with the colloids and with each other. The hydroxides will physically enmesh the colloids as they encounter them, creating a floc. The positive charge on the metal ion will neutralize the negative charge on the colloid. With the charge neutralized, the flocs can agglomerate until they reach a size that can easily fall out of solution.
There are several disadvantages to using metal salts for solid removal. First, the binding of water to the metal ions creates a gelatinous sludge with a high water content. Dewatering costs for metal ion sludges are typically higher than for other sludges. Second, the formation of metal hydroxides is pH dependent. If the pH of the system falls after the metal salts have been added, the hydroxides may not form and settling will not be enhanced. A third disadvantage to using metal coagulants is that a number of metals will form complexes or ligands with phosphate. Phosphate is essential to much bacterial activity. The formation of the metal-phosphate complexes may cause the phosphates to fall out of solution or otherwise become unavailable to bacteria. This can upset the biological function of the system.
Synthetic organic polymers are common coagulants and flocculents that are used in place of metal salts. These polymers are usually ionic, although some are nonionic. The ionic polymers capture the colloids through ion-ion interactions. The charge on the colloids is neutralized by the opposite charge of the polymer. The nonionic polymers capture colloids through polymer geometry, dipole ion interactions and van der Waals forces. The polymers neutralize the charge on the colloids, which overcomes the van der Waals force between the colloid and the water dipole. The polymers can also join several colloidal particles together through inter-particle bridging. Strands of polymer connected particles then become physically entwined in one another to form flocs. As other suspended particles come into contact with the floc, they become physically enmeshed. The flocs will grow by continued enmeshment of suspended particles and by agglomeration with other floes until they become dense enough to settle out.
While polymers generally work well in the removal of many suspended solids, there are several disadvantages to the use of synthetic polymers. First, synthetic polymer treatment is expensive, due in large part to the quantities that are required. Plants often must add as much as several hundred pounds of polymer per million gallons of flow treated per day.
Second, many of the polymers are toxic to aquatic species. When a polymer adheres to a biological floe and eventually settles out, it has not "disappeared." Instead it has adhered to the sludge, which will eventually have to be disposed. Changing conditions such as pH or temperature or other environmental factors can cause dissolution of the polymer and the release of its toxic constituents. Thus, sludges containing toxic polymers may need to be treated before they can be placed in a landfill or other appropriate disposal facility.
A third disadvantage that synthetic polymers face is their inflexibility. Many wastewater treatment plants have an influent that is not constant. Variance in the influent can change the properties required of the polymer. Thus, a polymer that can successfully coagulate the suspended solids present in the wastewater on Monday may not work on Wednesday.
A fourth disadvantage of synthetic polymers is their lack of effect on BOD.sub.5. If chlorination has been used to combat filamentous bacteria causing TSS problems, effluent BOD.sub.5 levels may become a problem. The prior art coagulants typically have no effect in this regard.
Accordingly a coagulant and flocculent and a method of using the same meeting the following objectives is desired